Gas dilution method and apparatus

ABSTRACT

A method and apparatus for preparing dilute HF for fluxless soldering, to provide very dilute anhydrous hydrogen fluoride to effect surface modification of the parts to be soldered. There are six main parts of the apparatus: a vacuum sub-system; a source gas sub-system; a dilution gas sub-system; a source gas/dilution make-up sub-system; a diluted gas storage sub-system; and a control system.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This invention is disclosed in Disclosure Document No. 460022, filed Jul. 28, 1999, with the U.S. Patent Office.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to methods for fluxless soldering, and in particular, relates to gas dilution method and apparatus allowing fluxless soldering with very dilute hydrogen fluoride.

[0004] 2. Description of the Related Art

[0005] Fabrication and assembly of electronic circuits requires that certain components of the circuit, such as integrated circuits (chips) or other components such as resistors, capacitors, etc., be in electrical contact with each other. They are for this reason mounted on substrates such as printed wiring boards so that the chip and substrate make electrical contact and are held together with the substrate supporting the component.

[0006] Soldering processes for connecting the components and the substrate generally involve pre-cleaning and deoxidation of surface oxides, generally using a liquid flux material, such as a weak organic acid applied prior to soldering and activated during the soldering process (e.g., at 230° C.) in a wave solder bath to prepare the surface to be soldered by removing contaminants including metal oxides from the solder surface. After the pre-cleaning, the next step of solder reflow and/or reflow joining can be performed after all oxides are removed from the solder surface because the oxides prevent the wetting of the two surfaces to be joined by the soldering reflow. When solder is heated it reflows and joins the surfaces which are in contact with solder. Finally, post-soldering cleaning removes the flux residue. This step is particularly difficult due to the small size of typical electronic components, and the difficulty of getting the cleaning agents to reach the minute areas between components.

[0007] Numerous investigators have attempted to eliminate the flux requirement for soldering. Fluxless soldering, or soldering without use of a liquid flux, replaces the first step with an alternate treatment, and can eliminate the third cleaning step. One example of fluxless soldering methods is the process of Dishon and Bobbio which utilizes plasma excitation using innocuous fluorinated gases, such as SF₆ or CF₄, to remove surface oxides from solder surfaces (U.S. Pat. No. 4,921,157). It is believed that these and other fluorinated gases of non-reactive substances were used as a source of plasma dissociated fluorine atoms, so that these atoms would react with and fluorinate the tin oxide (SnO₂) surface layer on the solder. Once this reaction occurs, the solder surface tension is sufficiently lowered so that joining is possible. In this method, the joining or reflow may be done any time within two weeks of the fluorination. The disclosure of this patent and all other patents and publications referred to herein is incorporated herein by reference.

[0008] Other more recent examples of fluxless soldering include wave soldering (U.S. Pat. No. 5,044,542 of Deambrosio), palladium enhanced fluxless soldering (U.S. Pat. No. 5,048,744 of Chang et al.), use of various inert gases (U.S. Pat. No. 5,139,193 of Todd), use of reducing agents such as lithium, calcium, strontium and cesium (U.S. Pat. No. 5,139,704 of Holland et al.), use of a laser beam (U.S. Pat. No. 5,164,566 of Spletter et al.), use of COHF (U.S. Pat. No. 5,071,486 of Chasteen), use of a heated reducing or non-reactive gas (U.S. Pat. No. 5,205,461 of Bickford et al.), and use of carbon-fluoride compositions (U.S. Pat. No. 5,380,557 of Spiro).

[0009] Most relevant to this invention is U.S. Pat. No. 5,609,290 of Bobbio et al. where it was found that use of HF, having very strong internal bonds and not dissociating easily to yield free fluorine, is an efficient method of fluxless soldering and provides a surface layer on the solder which allows fluxless reflow. In this work, anhydrous HF is reflowed directly over the parts to be soldered, without dilution (e.g., about 100% HF). Prior to the invention herein, it was thought that a high concentration of HF was required for enabling fluxless soldering after HF treatment.

[0010] There are problems, however, with using concentrated HF, including problems with disposal of this concentrated toxic gas, and potential problems due to the need to prevent leakage of the effluent to the environment. Also, at high HF concentrations, if there is moisture present, undesired etching of silicon oxide (SiO₂) or other metal oxides occurs. These oxides may be present on the parts to be soldered. Although water vapor can be minimized in an open environment by making sure the gases and treatment environment are dry plus the use of mild heating in the treating environment, it cannot be completely eliminated, and therefore oxide etching is a usual problem with the use of a high HF concentration as is known in the art.

[0011] It is therefore an object of the invention to provide a method and apparatus for fluxless soldering which uses anhydrous hydrogen fluoride (HF) at a very high dilution for surface modification of the parts to be soldered.

[0012] It is a further object of the invention to provide a method and apparatus that achieves the dilution of anhydrous HF in a safe and controlled manner.

[0013] Other objects and advantages will be more fully apparent from the following disclosure and appended claims.

SUMMARY OF THE INVENTION

[0014] The invention herein is a method and apparatus for preparing dilute HF for fluxless soldering, comprising providing very dilute anhydrous hydrogen fluoride to effect surface modification of the parts to be soldered. There are six main parts of the apparatus: a vacuum sub-system; a source gas sub-system; a dilution gas sub-system; a source gas/dilution make-up sub-system; a diluted gas storage sub-system; and a control system.

[0015] Other objects and features of the inventions will be more fully apparent from the following disclosure and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a schematic diagram of the apparatus of the invention.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS THEREOF

[0017] The method of the present invention provides a process for fluxless soldering which uses anhydrous hydrogen fluoride (HF) at a very high dilution for surface modification of parts that are to be soldered, so that the parts are solderable without the use of flux. The apparatus and method of the invention allow the anhydrous HF gas bottle to be isolated from the system except during short periods when the system is recharging its storage tank.

[0018] The method for fluxless soldering of the invention comprises exposing the parts to be soldered to a gas phase pretreatment of diluted anhydrous hydrogen fluoride in a dry carrier gas. The preferred concentration of anhydrous hydrogen fluoride is from about 30 ppm to about 100,000 ppm, with a preferred range of about 300 ppm to 30,000 ppm (0.03% to 3% HF). The amount of dilution of the HF that is optimum depends upon the avoidance of etching (or otherwise damaging) any metallic oxides that may be present consistent with efficacious fluxless treatment of the solder. Because the parts to be treated bring different amounts of water into the treatment region, the optimum amount of dilution therefore depends on the type of substrate or components being treated with the HF. Thus, the treatment of parts containing relatively large quantities of entrapped moisture within their structures (for example, porous material like FR4 fiberglass circuit board substrate material) shows a tendency to etch metallic oxides at lower HF gas concentrations than in the case of less porous materials, such as silicon substrates. The etching effect is not linear and for a given set of samples operation below a certain concentration leads to virtually no etching while operation above that level produces significant damage. Neither the fact that samples of practical interest (e.g. functional circuit boards) bring a damaging amount of water into the treatment region nor the fact that dilution of the hydrogen fluoride could effectively eliminate the damage were anticipated in prior work (Bobbio et al., U.S. Pat. No. 5,609,290).

[0019] The amount of dilution that may be used also depends on the ambient environmental conditions, such as relative humidity. In one instance, utilizing the method of the invention herein to supply diluted HF to a conveyor-driven atmospheric pressure treatment oven with dry dilution gas where glass epoxy circuit boards were also present, there was no metallic oxide etching at anhydrous HF concentrations up to 30,000 ppm. In a case where only test samples of silicon oxide on silicon wafers were present and dry dilution gas was used there was no etching at levels up to 100,000 ppm. Beyond this level, metallic oxide etching has always been observed. The metallic oxide etch rate determines the absolute maximum practical level of HF as well as the maximum within the suggested range of operation. The minimum levels are determined by the observation that the absolute minimum for which the treatment has led to fluxless reflow was 30 ppm on solder samples with no circuit boards present and 300 ppm for solder coated on glass epoxy boards.

[0020] The dry carrier gas is an inert gas, preferably dry air, dry nitrogen gas, dry argon or other inert gas preferably at a dew point below −40° C. The pressure during pretreatment is preferably at or near atmospheric pressure.

[0021] It is very important that the preparation of the diluted HF of the invention be controlled carefully and automatically, so that the anhydrous HF gas source for the fluxless treatment apparatus may be safely operated in a manufacturing environment. The method of building the apparatus and the method of operation of the apparatus to dilute the concentrated gas and deliver the diluted mixture to the fluxless processing tool are designed with this in mind.

[0022] Absolute pressure transducers as are known in the art are used to measure and control the diluted anhydrous HF delivery pressure and the anhydrous HF gas concentration. Adjusting the partial pressure of the anhydrous HF and dilution gas allows precise concentration control. One unique feature of this invention is that the highly concentrated HF is only connected to the process system for short periods of time. This feature provides a measure of safety due to the fact that there is only a limited source of potentially hazardous gas. In addition, using a flexible control, the concentration of the process gas can be controlled precisely between 0 to 100% concentrations.

[0023] System components. FIG. 1 shows the HF gas delivery apparatus of the invention. There are six main parts of the apparatus: (A) the vacuum sub-system; (B) the source gas sub-system; (C) the dilution gas sub-system; (D) the source gas/dilution makeup sub-system; (E) the diluted gas storage sub-system; and (F) the control sub-system. The apparatus of the invention provides diluted HF which can then be delivered to a process tool, such as a soldering system (not shown) as is known in the art.

[0024] The vacuum sub-system (A) removes residual gases from both storage tanks (described in more detail below) and the gas lines. This insures a repeatable starting point for establishing the desired dilutions of anhydrous HF. A mechanical vacuum pump 10 or other source of vacuum as is known in the art, exhausted by an exhaust connection 12, is connected via a pump inlet connection 14 to a first tank, make-up storage tank 16. An electropneumatic isolation valve 18 on the pump inlet connection 14 is normally in the closed position. A supply 20 of gas for purging, as described below, is connected to the mechanical vacuum pump 10 with connection 22. The vacuum subsystem also includes gas flow check valve 24, pneumatic cavity purge gas supply isolation valve 26 and control valve 28 on connection 22, the use of which is discussed below.

[0025] The source gas sub-system (B) delivers concentrated anhydrous HF to the first and second storage tanks, storage tanks 16 and 30 respectively, from a source gas supply 32. A source gas manual isolation valve 34 enables the flow of HF to be stopped manually. Source gas supply valve 36 is normally closed, and is only opened when concentrated HF gas is needed.

[0026] The dilution gas sub-system (C) delivers the dilution gas for the anhydrous HF, as discussed below, and is composed of a dilution gas supply 38, dilution gas supply flow check valve 40, and a normally closed dilution gas electropneumatic supply valve 42.

[0027] The source gas/dilution make-up sub-system (D) delivers the diluted anhydrous HF (a controlled mixture of gases from the source gas supply 32 and from the dilution gas supply 38) to the make-up storage tank 16 and provides the means to maintain this dilute gas mixture. This sub-system also includes a make-up storage tank electropneumatic isolation valve 44, which is normally closed, and an absolute pressure/vacuum transducer 46.

[0028] The diluted gas storage sub-system (E) is used to contain and deliver the diluted anhydrous HF gas mixture to the process tool containing parts to be soldered or otherwise treated, and includes a second storage tank, diluted gas storage tank 30, a storage tank electropneumatic isolation valve 48 that is normally closed, a diluted gas flow control valve 50, and a storage tank absolute pressure transducer 52, for control and measurement of the diluted gas pressure.

[0029] The control system (F) is composed of a dedicated controller (a personal computer, for example, IBM Aptiva Personal Computer, IBM Corporation, New York City; a programmable logic controller, for example, model SYSMAC C20K, Omicron Electronics Inc., Schaumburg, Ill.; a dedicated logic controller, for example, a Phase 4 Control System of IEI, Inc., Cary, N.C.; and a custom-built dedicated sequence controller designed to execute the system as detailed (e.g., a dedicated central processing unit (CPU) such as an Intel Single Board Computer equipped with a read only memory (ROM) program to execute the steps in sequential order, or other devices, as are known in the art for flow control. The control system monitors the two pressure transducers 46,52, operates all electropneumatic valves, and operates the flow control devices.

[0030] Dilution and delivery of the diluted HF are accomplished through operation by the control system of the pneumatic valves and flow control valves in a predetermined sequence. The preferred sequence of operation of the gas dilution system of the invention follows. Prior to initiation of use of the apparatus, all electropneumatic and manual valves are closed. The source gas supply 32, contains concentrated anhydrous HF gas; the dilution gas supply 38 contains, for example, dry air, dry nitrogen, dry argon, or other inert gas; and the cavity purge gas supply 20 contains, for example, dry air, dry nitrogen, dry argon, or other inert gas.

[0031] In FIG. 1, the wider lines connecting the sub-systems are preferably larger diameter pipes to facilitate high conductance during the evacuation of the two tanks 16,30 (discussed below), and valves 18, 44 are sized to match the line diameter as is known in the art (for example, if a ½-inch line is used, ½-inch or larger diameter valves should be used). The remainder of the lines shown can be sized according the desired flow. Thus, for up to 5 CFM (cubic feet per minute), ¼-inch diameter lines would be suitable, or sized for particular selected valves 24, 26, 28, 34, 36, 40, 42, 48, and 50. The exhaust connection 12 and the cavity purge connection 22 can be sized to match the selected vacuum pump (7) connections (typically ¼-inch to ⅜-inch for cavity purge connection 22 and 1 to 2 ½ inches for the pump exhaust connection 12. All gas and vacuum lines are fabricated from materials that are compatible with the gas chemistry. Thus, for using HF as a source gas, 316L is suitable. For the dilution gas, copper or 316L stainless steel is suitable. Additional lines as are known in the art, but are not shown separately in the figure would be part of the control sub-system connection to the remainder of the invention components, and are electrical wires or pneumatic lines to operate the automatic valves 18, 26, 28, 36, 42, 44, 48, and 50. The control sub-system selected and the type of valves selected would determine the nature of the lines between the control sub-system and the various automatic valves as is known in the art.

[0032] In the above description apparatus of the invention, several valves are mentioned as being preferably electropneumatic valves. Alternatively, electrical or pneumatic or electropneumatic valves, or a combination thereof may be used. The critical valve requirements are that the automatic valves can be remotely operated by a control subsystem and the valve construction and material of construction is compatible with the selected chemistry and pressures used in the apparatus. Examples of suitable valves are as follows: (a) valves 26, 36, 42, and 48: model SS-92M4-S4 (¼-inch diameter normally closed pneumatic toggle valve of stainless steel, Swagelok Co., Solon Ohio); (b) valves 18 and 44: model SS-45S8-33C (½-inch diameter normally closed pneumatic ball valve, made from stainless steel, Whitey Co., Highland Heights, Ohio); (c) valves 28 and 50: model 1179A Mass-Flo Controller (¼-inch diameter normally closed electronically controller variable flow valve, MKS Instruments, Andover, Mass.); (d) valves 24 and 40: model SS-6C-1 (¼-inch diameter 1 PSI flow check valve, made from stainless steel, Swagelok Co.); and (e) valve 34: model SS-4BKT-V51 (¼-inch manually operated isolation valve, made from stainless steel, Swagelok Co.). One of ordinary skill in the art would know how to substitute other suitable types and manufacturers for the above-examples of valves to assemble the apparatus of the invention.

[0033] The transducers used in the invention may be any suitable for the function as specified herein. Examples of transducers that may be used in the invention are: (a) transducer 46: model MKS Baratron Types 623A/624A/625A, absolute pressure transducer, 0-1000 Torr range, constructed of stainless steel, MKS Instruments, Andover, Mass.); and (b) transducer 52: model Pressure Transmitter Model 625-50 or 625-30 absolute pressure transducer, 0-30 psig or 0-50 psig range, constructed of stainless steel, Dwyer Instruments Inc., Michigan City, Ind.).

[0034] Although the above description of the apparatus of the invention utilizes only two tanks, the apparatus could be configured to supply several systems with diluted gas (at the same concentration or at different concentrations) by replicating the diluted gas storage sub-system E in a multi-delivery configuration, each replenished by a single arrangement of sub-systems A-D and F Initiation. To initiate apparatus operation, the operator opens the source gas manually operated shut-off source gas manual isolation valve 34. The mechanical vacuum pump exhaust connection 12 is connected to a suitable industrial exhaust system, as is known in the art.

[0035] Evacuation. The control sub-system is then used to open the mechanical pump electropneumatic isolation valve 18 and the make-up storage tank electropneumatic isolation valve 44, which connects the diluted gas storage tank 30 and make-up storage tank 16 to the mechanical vacuum pump inlet connection 14. Pressure transducers 46 and 52 are monitored by the control system. When a predetermined pressure set point is reached at the transducer 46 (e.g., 0.001 Torr to 10.0 Torr or 1.93×10⁵ PSIA to 0.193 PSIA (pounds per square inch absolute), the control system closes mechanical vacuum pump electropneumatic isolation valve 18. The purpose of evacuation is to remove gas present within tanks 16 and 30 prior to introduction of source gas and dilution gas. To accomplish this a low initial pressure is desired. The lowest value usable is a function of the vacuum pump 10 that is used. Typical vacuum pumps used for this apparatus achieve base pressures of approximately 0.001 Torr and thus the lowest expected value would be 0.001 Torr. The upper value of the range is determined by (a) the time required to achieve the desired starting lower pressure limit, which is variable and depends on the pumping speed of the vacuum pump 10, the size of the connecting lines and valves 18, 44, and the initial contents of tanks 16, 30; and (b) the desired accuracy of the diluted gas HF concentration. For example, if the final diluted gas storage tank 30 pressure is desired to be 24.7 PSIA and the evacuation set point of the make-up storage tank 16 is 0.001 Torr, the error in the final mix concentration would be approximately ±0.0081% of the expected concentration. For effective fluxless pretreatment using the diluted anhydrous HF, experimental results indicate that the concentration can vary as much as ±1.00% of the desired concentration. This implies that the upper limit on the evacuation set point can be 10 Torr.

[0036] Adding source gas to tanks. The control system opens the source gas supply valve 36, which connects the source gas supply 32 to the make-up storage tank 16 and the diluted gas storage tank 30. When the control system senses that pressure transducers 46 and 52 obtain a predetermined value (e.g., 2 Torr), the control system closes the source gas supply valve 36. The range of the transducer 46 determines the acceptable pressure range. Thus, with the preferred transducer, this range would be between 0 to 1000 Torr (0 to 19.3 PSIA). At this point, the make-up storage tank 16 and the diluted gas storage tank 30 contain a partial pressure (e.g., 2 Torr) of the concentrated source gas.

[0037] Adding dilution gas. Next, the control system is used to open the dilution gas electropneumatic supply valve 42 connecting the dilution gas supply 38 to the make-up storage tank 16 and the diluted gas storage tank 30. The dilution gas supply flow check valve 40 prevents gas back-flow into the dilution gas supply source 38. When the control system senses that pressure transducer 52 has reached a predetermined value, for example, 10 PSIG (pounds per square inch gauge, which is equal to PSIA minus atmospheric pressure, so that 10 PSIG is equal to 24.7 PSIA at sea level), the control system closes the dilution gas electropneumatic supply valve 42, isolating the dilution gas supply 38. The acceptable range of pressures at transducer 52 is between 0 PSIG to 50 PSIG. At this time, the diluted process gas mixture is in the make-up storage tank 16 and the diluted gas storage tank 30. For example, the values of 2 Torr of source gas and 10 PSIG of dilution gas would yield a diluted process gas concentration of 0.156% in the diluted gas storage tank 30. The replenished diluted gas should be at the concentration as determined only by the partial pressures of the P_(source)/P_(dilution) ratio, regardless of the size of the tanks.

[0038] Delivery of diluted gas-from diluted gas storage tank. The control system then closes the storage tank electropneumatic isolation valve 44. At this point, the diluted process gas is now ready for delivery. The control system accomplishes delivery by opening the storage tank electropneumatic isolation valve 48 and controlling the diluted gas flow control valve 50, and the diluted gas is delivered to the treatment system. The gas flows out to the treatment system due to a pressure differential between tank 30 and the pressure within the connected treatment system. The amount of flow is determined by the flow control valve 50.

[0039] Evacuating the make-up storage tank. In the next step, the control system connects the mechanical vacuum pump inlet connection 14 of the mechanical vacuum pump 10 to the make-up storage tank 16 by opening the electropneumatic isolation valve 18. When the control system detects that the storage tank pressure transducer 46 is below a predetermined value (e.g., 10 milliTorr), the control system isolates the make-up storage tank 16 from the mechanical vacuum pump inlet connection 14 by closing the mechanical vacuum pump electropneumatic isolation valve 18. During delivery of the diluted process gas via the storage tank electropneumatic isolation valve 48 and the diluted gas flow control valve 50, the control system monitors the storage tank pressure transducer 46.

[0040] Refilling of storage tanks. When the pressure within the diluted gas storage tank 30 drops below a predetermined value (e.g., 8.5 PSIG) as measured by the storage tank pressure transducer 46, the control system fills the make-up storage tank 16 with the source gas by opening the source gas supply valve 36. When the pressure within the make-up storage tank 16 reaches the predetermined value (e.g., 2 Torr) as measured by the make-up storage tank pressure transducer 52, the control system closes the source gas supply valve 36. Following this, the control system opens the dilution gas supply valve 42 and the make-up tank storage isolation valve 44. This introduces the dilution gas that is contained in the dilution gas supply 38 into both the make-up storage tank 16 and the diluted gas storage tank 30. The control system monitors the storage tank pressure transducer 52 while both tanks 16 and 30 are being refilled with the diluted process gas. When the control system detects that the storage tank pressure transducer 52 has reached a predetermined value (e.g., 10 PSIG), the control system closes the dilution gas supply valve 42 and the make-up storage tank isolation valve 44.

[0041] Repetition. The evacuation and refilling steps are repeated to maintain the diluted gas storage tank 30 pressure between predetermined points (e.g., 8.5 to 10 PSIG) at a fixed source gas concentration as determined by the control system programmable pressure set-points, as monitored by pressure transducers 46 and 52.

[0042] Cavity purge. During the operation of the gas dilution system, the mechanical vacuum pump 10 and the mechanical vacuum pump exhaust connection 12 are exposed to repeated cycles of potentially corrosive and/or hazardous gases from the source gas. To facilitate removal of any residual hazardous/corrosive gases and to extend the working lifetime of the mechanical vacuum pump 10 and the mechanical vacuum pump exhaust connection 12, a cavity purge is provided. The gas contained within the cavity purge gas supply 20 should be a gas chosen to provide a non-reactive environment for the selected source gas, such as dry nitrogen, dry air, dry argon or other inert gases. During the operation of the invention as described above, the control system opens the cavity purge gas supply isolation valve 26, and cavity purge gas flows through the cavity purge flow check valve 24 and through the cavity purge control valve 28 into the mechanical vacuum pump 10 through the cavity purge connection 22 and out the mechanical vacuum pump exhaust connection 12. This provides a continuous flow of a safe gas medium in the mechanical vacuum pump 10 and the mechanical vacuum pump exhaust connection 12, effectively sweeping any residual hazardous/corrosive source gas out of the mechanical vacuum pump 10 and the mechanical vacuum pump exhaust connection 12.

[0043] In addition to delivering the diluted HF formed by the method of the invention with the apparatus of the invention to a processing tool comprising solder on a surface, the method and apparatus of the invention may be used for treatment of solder powder which is used in solder paste. Conventional solder paste is a composite of solder powder and a vehicle which contains solder flux where the vehicle acts as a suspending medium for the powder. Solder paste may be applied to components and circuit boards in a number of ways including screen printing. During reflow, the flux dissolves surface oxides on the powder to allow the powder particles (now liquid) to flow smoothly together and to the parts to be joined. As with other conventional soldering processes, the flux leaves a residue which should be cleaned after joining. Using HF formed by the method of the invention also allows fluxless soldering with solder paste in which the solder powder has been pretreated using the methods described herein.

[0044] Preferably the entire apparatus is provided with a heating means and fan as is known in the art so that the apparatus may operate in a temperature range from ambient to 120 degrees C. in order to aid in maintaining a dry condition within the apparatus.

[0045] While the invention has been described with reference to specific embodiments, it will be appreciated that numerous variations, modifications, and embodiments are possible, and accordingly, all such variations, modifications, and embodiments are to be regarded as being within the spirit and scope of the invention. 

What is claimed is:
 1. A method for fluxless pretreatment of a substrate comprising exposing parts to be soldered to a gas phase pretreatment of diluted anhydrous hydrogen fluoride in a dry carrier gas, said hydrogen fluoride being diluted sufficiently to avoid metallic etching during the pretreatment.
 2. The method of claim 1, further comprising adjusting the dilution of the anhydrous hydrogen fluoride based on the substrate being pretreated and on ambient environmental conditions.
 3. The method of claim 1, wherein the concentration of the diluted anhydrous hydrogen fluoride is from 30 ppm to 100,000 ppm and preferably in the range 300 ppm to 30,000 ppm.
 4. The method of claim 1, wherein the dry carrier gas is an inert gas.
 5. The method of claim 4, wherein the dry carrier gas is selected from the group consisting of dry air, dry nitrogen gas, and dry argon.
 6. The method of claim 1, wherein the carrier gas has a dew point below −40° C.
 7. The method of claim 1, wherein the pressure during pretreatment is at or near atmospheric pressure.
 8. An apparatus for the delivery and dilution of hydrogen fluoride to a processing tool, comprising: (a) a vacuum sub-system for removing residual gases from the apparatus; (b) a source gas sub-system for supplying anhydrous hydrogen fluoride; (c) a dilution gas sub-system for delivering an inert dilution gas; (d) a source gas/dilution make-up sub-system for delivering the diluted anhydrous hydrogen fluoride gas mixture to a first storage tank and to a second storage tank to form a controlled diluted anhydrous hydrogen fluoride gas mixture, and for maintaining the controlled gas mixture in the second storage tank; (e) a diluted gas storage sub-system for containing and delivering the diluted anhydrous hydrogen fluoride gas mixture to the process tool to be soldered or otherwise treated; and (f) a control system for controlling pressure within the apparatus, flow of gases, and vacuum evacuation of the apparatus.
 9. The apparatus of claim 8, wherein the diluted gas storage sub-system comprises a flow control valve to deliver highly diluted hydrogen fluoride to the process tool.
 10. The apparatus of claim 8, wherein the vacuum sub-system comprises a mechanically operated vacuum pump.
 11. The apparatus of claim 8, wherein the inert dilution gas is selected from the group consisting of dry air, dry nitrogen, and dry argon.
 12. The apparatus of claim 8, wherein the vacuum sub-system further comprises a vacuum source and a supply of gas for purging the apparatus.
 13. The apparatus of claim 12, wherein the gas for purging the apparatus is selected from the group consisting of dry air, dry nitrogen, and dry argon.
 14. The apparatus of claim 8, wherein the concentration of the diluted anhydrous hydrogen fluoride gas is from 30 ppm to 100,000 ppm and preferably in the range 300 ppm to 30,000 ppm.
 15. The apparatus of claim 8, wherein the carrier gas has a dew point below −40° C.
 16. The apparatus of claim 8, wherein the entire apparatus is provided with a heating means so that the apparatus may operate in a temperature range from ambient to 120 degrees C. in order to aid in maintaining a dry condition within the apparatus. 